1. Field of the Invention
This invention relates to an ignition coil device for an internal combustion engine, and more particularly, to an ignition coil device for an internal combustion engine which is to be mounted in a plug hole of an engine body to apply high voltages directly to a spark plug.
2. Description of the Related Art
FIG. 4 shows a conventional ignition coil device for an internal combustion engine. In FIG. 4, reference numeral 21 denotes a cylindrical case made of an insulating resin, and a flat plate portion is integrally formed at the top of the case 21. Reference numeral 22 denotes a primary coil wound on a cylindrical bobbin 22A, the primary coil 22 being disposed in the case 21. Reference numeral 23 denotes a secondary coil wound on a cylindrical bobbin 23A, the secondary coil 23 being disposed inside the bobbin 22A for the primary coil 22. Reference numeral 24 denotes an iron core made of a magnetic material, the iron core 24 being inserted into the bobbin 23A for the secondary coil 23. Reference numeral 25 denotes magnets provided at upper and lower ends of the iron core 24, the magnets 25 being used for producing a bias magnetic field. Reference numeral 26 denotes a high-voltage output terminal attached to the lower end of the bobbin 23A for the secondary coil 23, and 27 denotes a coiled spring which is connected to the high-voltage output terminal 26 by screwing a screw 28 into a hole of the high-voltage output terminal 26.
The high-voltage output terminal 26 is connected to the secondary coil 23. Reference numeral 29 denotes a cylindrical shielding case with a slit, and 30 denotes a cylindrical insulating shell made of rubber. While being kept hot, an epoxy resin based insulating resin 31 is vacuum-injected into the case 21 housing the secondary coil 23, the primary coil 22, the iron core 24 and the magnets 25 so that the insulating resin 31 is filled in between the iron core 24 and the secondary coil 23, between the secondary coil 23 and the primary coil 22 and between the primary coil 22 and the case 21 to thereby enhance the insulating performance. Reference numeral 32 denotes an ignitor unit in which a power transistor, a switching element and so on are housed. The ignitor unit 32 is mounted at its lower portion on the flat plate portion of the case 21. The ignitor unit 32 is integrally formed with a connector portion and a mounting portion. By making use of holes in the mounting portion, the ignitor unit 32 is screwed to a cylinder head attached to the outer surface of an engine block.
When the ignition coil device shown in FIG. 4 is inserted into a plug hole of an engine body, a connecting portion of a spark plug is caused to come into an opening formed in the lower portion of the insulating cylindrical shell 30 made of rubber until the spring 27 is brought into contact with the connecting portion of the spark plug. In this condition, the ignitor unit 32 is screwed to the cylinder head by making use of the holes in the mounting portion of the ignitor unit 32.
In the state that the ignition coil device is inserted into the plug hole of the engine body as described above, the electric power and an on-off signal are supplied to a drive circuit and the switching element such as the power transistor housed in the ignitor unit 32 through the connector portion formed integrally with the ignitor unit 32 mounted on the case 21, so as to turn on/off the power transistor to thereby supply high ignition voltages to the spark plug.
Namely, when the switching element provided in the ignitor unit 32 of the above-described ignition coil device is turned off, the electric current flowing through the power transistor to the primary coil 22 is cut off and then high voltages are produced in the secondary coil 23 so as to be applied to the spark plug through the high-voltage output terminal 26, the screw 28 and the spring 27.
However, in the above-described conventional ignition coil device, since it is impossible for the compact type device that is to be mounted in the plug hole to increase the thickness of the layer of the insulating resin 31, and since the difference in thermal expansion between the bobbins 22A, 23A and the insulating resin 31 attributed to the temperature cycle causes separation to occur at the boundary surface, it is difficult to secure the withstand voltages of the primary coil and the secondary coil.